High-strength quenched formed body with good corrosion resistance and process for producing the same

ABSTRACT

A high-strength quenched formed body containing a layer on the surface of an after-quenching formed-body steel material in which layer Zn is a major component and which layer contains 30% by mass or less of Fe, and which layer is present in an amount of 30 g/m 2  or more. A quenched formed body is produced by quenching a zinc-plated steel material which includes a zinc-plated layer containing each of Al and Si having alloying-retarding function and readily-oxidizing function independently or compositely, in an amount of 0.15% by mass or more, after heating it to 800° C. or more and 950° C. or less in an oxidizing atmosphere containing 0.1% by volume or more of oxygen.

TECHNICAL FIELD

The present invention relates to a formed body, which is good in termsof the corrosion resistance and which is completed by performing aquenching process for the purpose of highly strengthening it, and aprocess for producing the formed body.

BACKGROUND ART

Recently, for the purpose of automotive lightening and safetyimprovement, it has been getting under way to highly strengthenautomobile component parts and raw materials employed for the same.Steel plates, one of the representative examples, too, are such that theemployment ratio of high-strength steel plates has about come toheighten. However, because of the fact that high-strength steel platesare of high strength and are hard in general, the degree of formingfreedom is small in the press formability; moreover, there are suchproblems that the configurational freezability of pressed products ispoor, the dimensional accuracy of formed products is defective and thelongevity of pressing dies is short. Against these assignments, theimprovements, which begin with the raw materials, have been gettingunder way as well. Recently, for the purpose of obtainingmuch-higher-strength component parts while providing them with goodconfigurational accuracy, hot-working or hot-pressing technologies havebecome widespread, hot-working or hot-pressing technologies in which asteel plate is heated to 800° C. or more to soften it, is cooled rapidlysimultaneously with press forming, and is quenched to make ahigh-strength component part; moreover, cold working-quenchingtechnologies have come to be employed as an industrial technology, coldworking-quenching technologies in which it is similarly quenched to makea high-strength component part after it is cold worked.

Meanwhile, since industrial machines, which are represented byautomobiles, are such that the corrosion resistance in serviceenvironments is required sufficiently, components has been employedcurrently, component parts which are made by cold forming zinc-basedplated steel plates, which are low cost and are good in terms of thecorrosion resistance; however, in addition to this, many inventions havebeen known publicly, inventions in which surface-treated steel materialsare heated to quench them.

In Patent Literature No. 1, a production method for a high-strengthformed component part is disclosed, high-strength formed component partin which a zinc or zinc-alloy coating film is formed in a thickness of 5μm-30 μm on a steel plate so that the protection against corrosion anddecarburization, and the lubricational function are secured. In PatentLiterature No. 2, a steel plate for hot pressing is disclosed, steelplate in which a barrier layer, which inhibits the volatilization ofzinc upon heating, is formed on a zinc-plated layer before heating forquenching treatment. In Patent Literature No. 3, a hot-pressing methodfor a zinc-system-plated steel plate is disclosed. In Patent LiteratureNo. 4, a hot-pressed formed product in which an iron-zinc solid-solutionlayer exists is disclosed.

However, in accordance with these methods, although all of them arebetter in terms of the corrosion than that of molded products, which aremade by subjecting plating-free iron to quenching treatment, thecorrosion resistance is still insufficient compared with that of moldedproducts, which are made of plated steel plates being formed by ordinarycold working. It is because zinc volatilizes by means of heating.Against this problem, although aluminum-plated steel plates have beenemployed for applications in which the corrosion resistance, beingequivalent to that of ordinary plated steel plates, is required, notonly their costs are high but also the after-quenching corrosionresistance lowers more than that of cold-formed members none the less.

In accordance with aforementioned Patent Literature No. 2, before thequenching treatment, a barrier layer, which comprises an oxidizedcoating film, is formed on the zinc-plated layer of the steel material.In this case, when heating the steel material to a quenchingtemperature, or when heating it to the quenching temperature and holdingit thereat, there is a fear that cracks might generate in thezinc-plated layer considerably because of the thermal expansiondifference between the barrier layer, which has been formed originallyon the steel material, and the zinc-plated layer. In this case, due tothe cracks, the fear that the volatilization amount of zinc increases ishighly likely, and it is not necessarily sufficient in order to obtain aplated layer whose corrosion resistance is good after the quenchingtreatment.

Against these problems, a technique has been desired strongly, techniquewhich makes it possible to highly strengthen quenching and improvecorrosion resistance in zinc-system-plated steel materials, which aremore predominant in view of cost.

Patent Literature No. 1: Japanese Unexamined Patent Publication Gazette(KOKAI) No. 2001-353,548

Patent Literature No. 2: Japanese Unexamined Patent Publication Gazette(KOKAI) No. 2003-73,774

Patent Literature No. 3: Japanese Unexamined Patent Publication Gazette(KOKAI) No. 2003-126,920

Patent Literature No. 4: Japanese Unexamined Patent Publication Gazette(KOKAI) No. 2003-126,921

The present invention, in view of the aforementioned problems, is forproviding a high-strength quenched formed body, which is good incorrosion resistance, in formed-body steel materials, to which thezinc-system-plated steel material, being predominant cost-wise, isperformed and in which the corrosion resistance of after-quenchingformed-body steel material is made equivalent to or more than that ofcold-formed product, and a production process for the same.

DISCLOSURE OF THE INVENTION

The present inventors, first of all, investigated earnestly the causesof why the corrosion resistance of the zinc-system-plated steel materialis inferior to that of ordinary zinc plated steel material, forinstance, that of alloyed molten zinc plated steel material, after thehot working at 800° C. or more being required for quenching it. As aresult, they reached the conclusion that the cause of why the corrosionresistance is poor is not only because Zn volatilizes so that the platedamount decreases but also because Zn, which constitutes the zinc-platedlayer, solves into Fe so that Fe turns into an Fe—Zn alloy layer, inwhich Fe is a major component. Namely, an ordinary zinc-plated steelmaterial is such that the corrosion resistance is demonstrated more bymeans of the effect that Zn, which is oxidized upon corrosion, turnsinto a dense protective film than by means of the sacrifice corrosionprevention. However, since the zinc-system-plated steel material, whichis hot worked at 800° C. or more, is such that an Fe—Zn alloy layerwhose Fe % is great is formed, the corrosion resistance is notdemonstrated, though the Zn content is present more superfluouslyquantitatively in the steel-material surface than it is in an ordinaryzinc-system-plated steel material. They considered that this is becauseof the fact that an Fe—Zn alloy layer, which is generated by means ofquenching, is such that Fe becomes a major component, and thereby theoxidized film of Zn cannot become a dense film due to the volumetricexpansion of Fe, which is oxidized upon quenching. Therefore, thepresent inventors arrived at completing the present invention based onthe view that, in order to have the corrosion resistance demonstrated,it is more important that a Zn—Fe alloy layer, in which Zn is a majorcomponent so that the quality is good (Fe % is less), is presentsufficiently in view of quantity as well.

A high-strength quenched formed body of the present invention accordingto a first aspect, high-strength quenched body which is good incorrosion resistance, is characterized in that it comprises anafter-quenching formed-body steel material; and a layer being disposedon the surface of the formed-body steel material, the layer beingsubjected to a quenching treatment along with the formed-body steelmaterial, the layer being made from Zn as a major component, and thelayer being formed of Fe: 30% by mass or less; and the layer, in whichZn is a major component and which is formed of Fe: 30% by mass or less,is contained in an amount of 30 g/m² or more. Note that the % in thepresent description means % by mass.

Here, the phrase, “Fe: 30% by mass,” means Fe % in such an instance thatthe layer is taken as 100% by mass. “m²” means per 1 m², the unitsurface area of the formed-body steel material.

The “layer in which Zn is a major component and which is formed of Fe:30% by mass or less” is a layer in which Fe is less but Zn is more, andhas high corrosion resistance. Even when a zinc-plated layer undergoes aquenching treatment along with a formed-body steel material, as far asthe “layer in which Zn is a major component and which is formed of Fe:30% by mass or less” is present in a predetermined value (30 g/m²) ormore, the corrosion resistance of the quenched formed body becomessatisfactory.

A process of the present invention according to a second aspect of thepresent invention for producing a high-strength quenched formed body,which is good in corrosion resistance, is characterized in that thefollowing are performed sequentially: a step of preparing a formed-bodysteel material, on which a zinc-plated layer is coated, zinc-platedlayer which contains each of Al and Si, which have alloying-retardingfunction and readily-oxidizing function, independently or compositely inan amount of 0.15% by mass or more; and a step of quenching theformed-body steel material after heating it to 800° C. or more and 950°C. or less in an oxidizing atmosphere of oxygen: 0.1% by volume or more,thereby producing:

a high-strength quenched formed body comprising: an after-quenchingformed-body steel material; and a layer being disposed on the surface ofthe formed-body steel material, the layer being subjected to a quenchingtreatment along with the formed-body steel material, the layer beingmade from Zn as a major component, and the layer being formed of Fe: 30%by mass or less; and the layer, in which Zn is a major component andwhich is formed of Fe: 30% by mass or less, is contained in an amount of30 g/m² or more.

Here, the phrase, “0.15% by mass or more,” means the amount ofreadily-oxidizing element (Al and/or Si) in such an instance that thebefore-quenching zinc-plated layer is taken as 100% by mass.

Usually, in 800-° C.-or-more hot working, since Zn has a sufficientvapor pressure, and since it vaporizes off within a heating furnace, theZn amount of zinc-plated layer is less likely to be maintained. Hence,in a before-heating zinc-plated layer upon quenching, by having thereadily-oxidizing element (Al and/or Si) contained in an amount of 0.15%by mass or more in the before-quenching-treatment zinc-plated layer, andby making the furnace's inside an oxidizing atmosphere in which0.1%-by-volume-or-more oxygen is present, the readily-oxidizing elementis oxidized continuously in the surface of the zinc-plated layer whilesuppressing the volumetric change resulting from heating, andaccordingly it is possible to form a dense oxidized film. Consequently,even when heating it for quenching in a heating temperature range of thequenching temperature region (800° C. or more and 950° C. or less), thesuppression of the volatilization of Zn in the zinc-plated layer becomespossible.

In accordance with the method of the present invention like this, sinceoxides grow gradually in the zinc-plated layer along with the initiationof heating for quenching treatment, the volatilization of zinc issuppressed. Accordingly, after the quenching treatment, the “layer inwhich Zn is a major component and which is formed of Fe: 30% by mass orless” can be obtained in a predetermined value (30 g/m² or more)satisfactorily, and thereby the corrosion resistance can be secured.

By the way, it is possible as well to think of a measure of forming abarrier layer, which is made of an oxidized film, on the zinc-platedlayer of steel material. However, in this case, when heating a steelmaterial to a quenching temperature, or when heating it to the quenchingtemperature and holding it thereat, there is a fear that cracks mightgenerate in the zinc-plated layer considerably because of the thermalexpansion difference between the barrier layer, which is formedoriginally on the steel material, and the zinc-plated layer. In thiscase, due to the cracks, the fear that the volatilization amount of zincincreases is highly likely. Therefore, it is inferred that, after thequenching treatment, it is not sufficient in order to obtain the layerwhose corrosion resistance is good (layer in which Zn is a majorcomponent and which is formed of Fe: 30% by mass or less) in thezinc-plated layer.

EFFECT OF THE INVENTION

In accordance with the present invention, it is possible to maintain thezinc content in the zinc-plated layer abundantly even when beingsubjected to quenching treatments. Accordingly, after-quenchingformed-body component parts are made so that the corrosion resistance,which is equal to or more than that of zinc-system-plated steelmaterials for cold-worked molded products, zinc-system-plated steelmaterials which have been employed in automobiles or industrial machinesconventionally, can be obtained, and additionally the dimensionalaccuracy of high-strength component parts can be improved remarkably,too. Therefore, it becomes possible to advantageously push forward theweight saving, safety improvement and rust-prevention improvement ofautomobiles and industrial machines, and accordingly the industrialcontribution is great extremely.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory diagram for illustrating an electrolyticcome-off curve in Comparative Example No. 2.

FIG. 2 is an explanatory diagram for illustrating an electrolyticcome-off curve in Comparative Example No. 9.

FIG. 3 is an explanatory diagram for illustrating an electrolyticcome-off curve in Example No. 6.

FIG. 4 is an explanatory diagram for illustrating an electrolyticcome-off curve in Comparative Example No. 1.

FIG. 5 is an explanatory diagram for illustrating an electrolyticcome-off curve in Comparative Example No. 10.

FIG. 6 is an explanatory diagram for illustrating an electrolyticcome-off curve in Example No. 9.

FIG. 7 is an explanatory diagram for illustrating an electrolyticcome-off curve in Example No. 10.

FIG. 8 is an explanatory diagram for illustrating an electrolyticcome-off curve in Comparative Example No. 5.

BEST MODE FOR CARRYING OUT THE INVENTION

A high-strength quenched formed body of the present invention comprisesan after-quenching formed-body steel material, and a layer beingdisposed on the surface of the formed-body steel material, the layerbeing subjected to a quenching treatment along with the formed-bodysteel material, the layer being made from Zn as a major component, andthe layer being formed of Fe: 30% by mass or less. The “layer in whichZn is a major component and which is formed of Fe: 30% by mass or less”is such that the corrosion resistance is good, and is contained in anamount of 30 g/m² or more.

Here, in the “layer in which Zn is a major component and which is formedof Fe: 30% by mass or less,” when the layer is taken as 100% by mass, Zncan preferably be contained in an amount of 70% by mass or more, 80% bymass or more, or 90% by mass or more. Even when undergoing a quenchingtreatment, if the plated layer is Zn-rich, high corrosion resistance canbe secured.

The high-strength quenched formed body of the present invention, whichis good in corrosion resistance, can be obtained by heating azinc-plated steel material, which comprises a zinc-plated layer whichcontains each of Al and Si, which have alloying-retarding function andreadily-oxidizing function, in an amount of 0.15% by mass or moreindependently or compositely in an oxidizing atmosphere of oxygen: 0.1%by volume or more in a temperature rage of 800° C. or more and 950° C.or less while adjusting the heating time appropriately; and thereafterquenching it. As for the oxidizing atmosphere, it is possible toexemplify oxygen: 0.1% by volume or more, oxygen: 1% by volume or more,or oxygen: 20% by volume or more.

Moreover, prior to the heating upon quenching, when the zinc-platedlayer is taken as 100% by mass, the zinc-plated layer contains each ofthe elements, Al and Si, independently or compositely in an amount of0.15% by mass or more. It is preferable as well to contain Mg, Ti or arare-earth element (Ce) as a readily-oxidizing element. Here, theelement, which has alloying-retarding function, means an element, whichretards iron, constituting the base metal of steel material, fromdiffusing into the zinc-plated layer. In this case, since the iron ofsteel material is retarded from diffusing into the zinc-plated layer, itis possible to lower the iron % in the zinc-plated layer. Moreover, thereadily-oxidizing element means an element, which is likely to generatean oxidized film. Note that, since Al and Si are readily-oxidizingelements and simultaneously alloying-retarding elements, they candemonstrate both functions, the readily-oxidizing function and thealloying-retarding function. Note that, when Al and Si are excessive,the Zn amount in the plated layer decreases relatively.

Here, when Al and Si are contained independently or compositely in theaforementioned before-quenching-treatment plated layer, it is possibleto exemplify 0.16% by mass or more, 0.18% by mass or more, 0.20% by massor more, 0.30% by mass or more, 0.40% by mass or more, 0.60% by mass ormore, and the like, as for the lower limit value. As for the upper limitvalue of the aforementioned elements (Al, and Si) which are combinablewith those lower limit values, it is possible to exemplify 3% by mass orless, 4.5% by mass or less, 4.7% by mass or less, 4.9% by mass or less,6% by mass or less, and 20% by mass or less.

Further, other than the above-described Al and Si, it is preferable tohave one member or two members or more of Mg, Ti and rare-earth elementsincluded as the readily-oxidizing element in the zinc-plated layer. Inthis case, it is possible to make the oxidized film much firmer, and itbecomes possible as well to suppress the volatilization of Zn more.Therefore, the before-quenching-treatment zinc-plated layer is such thatit is possible to exemplify modes, which contain one member or twomembers or more of Mg, Ti, rare-earth elements (Ce, and the like), Fe,Ni and Co in an amount of 3.0% by weight or less, or 2.0% by mass orless.

When the aforementioned readily-oxidizing elements are less than 0.15%by mass, or in such a neutral atmosphere and a reducing atmosphere thatthe in-furnace atmosphere is less-than-0.1%-by-volume oxygen, no densefilm of readily-oxidizing element can be formed sufficiently. In thiscase, the vaporizing-off of Zn is done, and accordingly the Zn amountfor rustproofing is decreased. Moreover, when the heating temperature ismuch lower than the aforementioned temperature region, although it isadvantageous for the volatilization prevention of Zn, the quenching forobtaining the high-strength formed body, the original object, cannot bedone sufficiently. When the heating temperature is much higher than theaforementioned temperature region, even with the oxidized film whichresults from the readily-oxidizing elements, the vaporizing-off by meansof the boiling of Zn cannot be suppressed.

In accordance with the present invention, it is possible to exemplify amode in which heating upon quenching is carried out while growing theoxidized film. Further, in order to make the corrosion resistance equalto or more than that of the ordinary zinc-plated layer, the “Zn—Fe alloylayer in which Zn is a major component and which is formed of Fe: 30% bymass or less” should be made in an amount of 30 g/m² or more. In orderto do so, as the alloying-retarding element, it is preferable to containeach of metals, which are formed of Al and Si which double as thereadily-oxidizing element, independently or compositely in an amount of0.15% by mass or more.

When the aforementioned elements are present in an amount of 0.15% bymass or more in the zinc-plated layer in the instance prior to theheating for quenching, even upon being heated to a high temperature of800° C. or more, it is possible to control the diffusion of Zn into thebase iron remarkably. As a result, even after the quenching treatment,it is possible to make the “Zn—Fe alloy layer in which Zn is a majorcomponent and which is formed of Fe: 30% by mass or less” in an amountof 30 g/m² or more. In this case, it can be set at 40 g/m² or more, 50g/m² or more, 80 g/m² or more, 100 g/m² or more, 200 g/m² or more, 250g/m² or more, and the like.

On the contrary, when the aforementioned elements are less than 0.15% bymass, the diffusion of Zn into the base iron of steel material is toofast; the “Zn—Fe alloy layer in which Zn is a major component and whichis formed of Fe: 30% by mass or less” disappears virtually until thetemperature of steel material reaches 800° C.; and accordingly thefavorable corrosion resistance cannot be demonstrated. Note that theupper limit value of the concentration of alloying-retarding elements inthe zinc-plated layer can be in such a range that does not change thisZn—Fe alloy layer, because they are added in order to obtain the “Zn—Fealloy layer in which Zn is a major component and which is formed of Fe:30% by mass or less”; taking the cost, too, into account, it issufficient that, when the before-quenching zinc-plated layer is taken as100% by mass, the alloying-retarding elements can be 20% by mass orless.

Regarding the holding time for holding it at the heating temperature,although it depends on the thickness (plate thickness) of steel material(steel plate) to be heated, the oxygen concentration, and the like, itcan be such a temperature that a steel material as a whole reaches atemperature required for quenching. For example, it is possible toexemplify 1 second-50 minutes, and 10 seconds-20 minutes. In general,the thinner the thickness of steel material is the shorter the heatingtime is, and the thicker the thickness of steel material is the longerthe heating time is. When the temperature-ascending time for ascendingthe temperature to a target temperature is appropriate, the holding timecan be either 0 second or 0 minute.

Moreover, in the case where the heating time is prolonged by means ofthe thickness -of steel material, a furnace length and a handlingapparatus, it is preferable to increase the alloying-retarding-elementamount per unit area in the plated layer (the concentration increment ofthese alloys in the plated layer or the increment of plated amount).

With regard to the heating method, it can be any one of internalheating, such as induction heating, external heating, such as infraredheating, gas heating and electric furnaces, and using these combinedlyfor shortening the heating-time. Note that, as for the plated amount ofZn, although it depends on aiming corrosion-resistance targets, it canbe 40 g/m² or more; however, when taking the time for the handling ofheating furnace and the temperature fluctuation into consideration, itcan preferably be 60 g/m² or more; when taking the running of Zn, whichresults from the disposition of steel material within a furnace, intoconsideration, it can preferably be 300 g/m² or less. In particular, itcan preferably be 180 g/m² or less so that the occurrence of running ishardly appreciated even when it is disposed perpendicularly.

The aforementioned zinc-plated layer, as it can be apparent from theaforementioned principle, can be those prepared by a molten zinc platingmethod in which a zinc-plated layer is adhered to it by immersing itinto zinc in molten state. Although it is possible to employ alloyedmolten zinc-plated-materials, which have been alloyed in advance, theyresult in the superfluous disappearance of the alloying-retardingelements (Al, Si, and the like), and accordingly the effects tends toreduce. Moreover, although an electric zinc plating method, too, can beemployed, it is costly because a pre-treatment, and so forth, is neededfor the addition of the alloying-retarding elements (Al, Si, and thelike).

Next, although it is on the quenching, it can be done at a cooling ratewhich makes quenching possible; it can be any one of methods, such aswater cooling the formed-body steel material, cooling by means ofcontact with die, and gas cooling. As for the cooling by means ofcontact with die, die quenching can be named. By means of quenching,quenched structures can be obtained.

Note that, although it is on the mode of the present method, it can beany one of [1] a zinc-plated-system steel material is heated and thencooled after it is cold worked, [2] a steel material is heated and thencooled after it is cold worked and then subjected to zinc plating, [3] azinc-plated-system steel material is worked and then cooled after it isheated like so-called hot pressing.

Although a configuration as the formed-body steel material is notlimited in particular, it is possible to exemplify platelike shapes, andaggregated shapes. As for the platelike thickness, although it is notlimited in particular, it is possible to exemplify 1 mm or less, 2 mm orless, 5 mm or less, and 10 mm or less; however, it is not limited tothese. As for the carbon content of the formed-body steel material, itcan be hardenable amounts; when taking the formed-body steel material as100% by mass, 0.01-0.9% by mass, 0.1-0.7% by mass, and 0.1-0.4% by masscan be exemplified; however, it is not limited to these.

Next, it is on the components of the steel material, any one of ordinaryhardenable steel materials is allowable. In general, although thefollowing steel-material components can be exemplified, they are notlimited to these. The steel-material components, as the composition ofthe steel material, contain C: 0.01-0.30%, Si: 0.005-1.0%, Mn:0.01-3.0%, P: 0.005-0.10%, S: ≦0.02%, and N: 0.001-0.01%, by mass %.Note that, as the steel-material components, it is preferable to containB: 2-100 ppm, Cr: 0.02-0.500%, Mo: 0.001-0.500%, and Ni: 0.001-1.000%;further, as the compositional elements of the steel material, it ispreferable to contain one member or two members or more selected fromthe group of Nb: 0.005-0.100%, V: 0.005-0.100%, Ti: 0.005-0.100%, andZr: 0.005-0.100%.

Next, the after-quenching formed-body steel material will be described.In accordance with the present invention, there should be the “Zn—Fealloy layer in which Zn is a major component and which is formed of Fe:30% by mass or less” in an amount of 30 g/m² or more in the surface ofthe after-quenching formed-body steel material. When being less than 30g/m², no sufficient corrosion resistance can be obtained because thealloy layer, which is generated by means of heating upon quenching andin which Fe is a major component, generates Fe rust upon corrosion andexpands volumetrically. Note that, with regard to the Zn—Fe layer, whichis generated by means of heating and in which Fe is a major component,although no limitation is set up therefor especially, in the presentinvention, it often generates in amount of 5 g/m² or more in general.The after-quenching strength can be such that needed strength isavailable; although it depends on purposes, it can be 800 MPa or more.

Note that, after the quenching treatment, even when the superficialoxidized film is removed with an alkaline liquid or an acidic liquid forthe purpose of the improvement of coating adhesiveness or chemicaltreatability, it falls within the scope of the present invention as faras it is possible to let the “Zn—Fe alloy layer in which Zn is a majorcomponent and which is formed of Fe: 30% by mass or less” exist.Moreover, even when having an element, such as Ni, Co, Mn, P and B,contained in the Zn—Fe alloy layer (alloy-plated layer) for the purposeof the further improvement of corrosion resistance or the improvement ofchemical treatability, it falls within the scope of the presentinvention as far as Zn is the major component and it is formed of Fe:30% by mass or less.

EXAMPLES

Examples of the present invention will be named along with comparativeexamples. As for the steel material (formed-body steel material),hot-rolled steel plates and cold-rolled steel plates, which wereproduced by ordinary production processes, were used. Table 1 specifiesthese steel components. And, zinc-plated layers were laminated onto thesteel materials, thereby forming test pieces. Table 2 specifies the dataon zinc-plated layers, and the data on the heating conditions ofquenching treatments. Table 3 specifies the after-quenching-treatmentdata. Here, Table 2 specifies the steel species of test pieces, theplate thicknesses of test pieces, the plating conditions (platingmethods, plated amounts, inner Zn amounts, the compositions of platedlayers, the plating species of upper-layer plating, the plated amountsof upper-layer plating, and the heating conditions in quenchingtreatments (heating methods, heating temperatures, holding times,heating atmospheres, and oxygen concentrations)). Here, the “holdingtime” means the time for holding them at a heating temperature, a targettemperature; and, when a test piece is quenched immediately after thetemperature is increased to a heating temperature, the holding timebecomes 0 minutes. The “air ratio=1” in the heating atmosphere means onefold of theoretical combustion amount. The “air ratio=1.1” means 1.1times of theoretical combustion amount.

Table 3 specifies the cooling methods in quenching treatments, the dataon “Fe<30% Zn—Fe alloy layer,” the data on “30%<Fe Zn—Fe alloy layer,”the Zn volatilization amounts, the corrosion resistance (swollen width),and the strength. Here, “Fe<30% Zn—Fe alloy layer,” specified in Table3, is equivalent to a “layer in which Zn is a major component and whichis formed of Fe: 30% by mass or less (equivalent to later-describedportion “A”).” Moreover, “30%<Fe Zn—Fe alloy layer” is equivalent to a“layer in which Fe is a major component and is beyond Fe: 30% by mass(equivalent to later-described portion “B”).”

Here, it is difficult to carry out the addition of readily-oxidizingelement and alloying-retarding element to a plated layer byelectroplating. Accordingly, a predetermined amount of readily-oxidizingelement and alloying-retarding element was added to a molten Zn platingbath, and it was carried out by an ordinary molten Zn plating method.Note that, with regard to the steel species (“D” in Table 1), in whichSi>0.2%, Mn>1.5% and B>15 ppm, since the plating wettability isinsufficient, after forming an Fe plating in an amount of 5 g/m² asundercoated plating by electroplating, a molten Zn-plated layer wasformed on the undercoated plating (Example No. 28 and ComparativeExample No. 13).

With regard to the addition of Fe, Ni and Co to the Zn-plated layers, itwas carried out in the following manner. That is, ready-made platingbaths mentioned below were used; prior to quenching, upper-layerelectroplating was performed onto the top of molten zinc plated layers.Thereafter, it was carried out by diffusing Fe, Ni and Co intozinc-plated layers by means of heating upon quenching (Example Nos.23-26). As for the upper-layer electroplating, it was done as follows.

Electro-Fe-plating: Ferrous Sulfate Plating Bath

Electro-Ni-plating: Watt Bath

Electro-Co-plating: Cobalt Sulfate Plating Bath

Note that, in accordance with the present example, the treatment foractively forming a barrier layer on a zinc-plated layer was notperformed prior to the quenching treatment.

With regard to the quenching treatment, based on the heating conditionsspecified in Table 2, the steel plates were heated up to heatingtemperatures within an electric furnace, or a high-frequency inductionheating furnace, or gas furnace, or an infrared heating furnace, in anair atmosphere, or in predetermined-air-ratio atmospheres. Thereafter,the steel plates were taken out of the furnaces; and then, based on thecooling methods specified in Table 3, the steel plates were quenched bymeans of water cooling, or die cooling, or gas cooling, thereby carryingout quenching.

In accordance with the present examples, with regard to the “Fe<30%Zn—Fe alloy layer,” it was analyzed as follows. That is, the test pieceshaving zinc-plated layers were placed in a 150-g/liter ammonium chloride(NH₄Cl) aqueous solution while using a saturated calomel electrode as areference electrode; and then the plated layers (layer “G”: the portion“A” of the drawings in examples and comparative examples) were come offdown to the layer “G,” at which the potential changed greatly to apotential of −800 mV (vs. SCE), by electrolysis by means ofconstant-current electrolysis with 4 mA/cm² at room temperature. Here,“−800 mV (vs. SCE)” means a negative-side potential, a potential whichwas lower than the potential of saturated calomel electrode by 800 mV.This electrolyte includes components resulting from the dissolvedportion “A.” The electrolytes, into which the portion “A” thusdissolved, were analyzed by means of an ICP (Inductively Coupled Plasma)analyzing apparatus. By means of this, with regard to the “layers inwhich Zn is to a major component and which are formed of Fe: 30% by massor less,” the Fe amounts, the Zn amounts, and the compositional ratioswere found; and then the alloy-layer amounts (g/m²), the inner Znamounts (g/m²), and the Fe % s were found, as the plated amounts, whichexhibited rust-preventive effect. They were specified in Table 3.

Moreover, with regard to the measurement of “30%<Fe Zn—Fe alloy layer,”it was electrolyzed down to the aforementioned layer “G” to come off theportion “A” of zinc-plated layer; and thereafter the electrolyte wasreplaced with a new solution; and it was electrolyzed up to thepotential of iron (about −560 mV (vs. SCE)) (equivalent to the portion“C” of the drawings in examples and comparative examples) continuously.This electrolyte includes components resulting from the dissolvedportion “B.” And, the electrolytes were analyzed by means of an ICPanalyzing apparatus similarly; the Fe amounts, the Zn amounts, and thecompositional ratios were found; and then the alloy-layer amounts(g/m²), the inner Zn amounts (g/m²), and the Fe %s were found. They werespecified in Table 3. Here, in Table 3, the “alloy-layer amount g/m²,”which is set forth in the columns of “Fe <30% Zn—Fe alloy layer” and“30%<Fe Zn—Fe alloy layer,” means the total amounts of alloyingelements, such as Zn, Fe, Al and Si, which were contained in Zn—Fealloys. Moreover, in Table 3, the “Zn volatilization amount” was suchthat the differences between the before-heat-treatment Zn amounts andthe after-heat-treatment Zn amounts were measured by means of ICP.

In Table 3, in accordance with Example Nos. 1-28, the “layer in which Znis a major component and which is formed of Fe: 30% by mass or less(equivalent to the portion “A”)” was made in an amount of 30 g/m² ormore after quenching treatment. In this layer, the inner Zn amount wasmade in an amount of 14 g/m² or more. The “inner Zn amount” means a Znamount in the “layer in which Zn is a major component and which isformed of Fe: 30% by mass or less (equivalent to the portion “A”).”

With regard to the strength, in order to make the after-productionstrength evaluation tougher, test pieces (JIS #5 tensile test piece)were cooled by means of gas jet; and thereafter they were evaluated bystretching them in the “L”-direction. The evaluation results arespecified in Table 3. Those, which surpassed 800 MPa, were evaluatedbeing satisfactory. In accordance with Example Nos. 1-28, they surpassed800 MPa.

With regard to the corrosion resistance, the test pieces were evaluatedin the following manner: degreasing was carried out, and a chemicaltreatment was carried out onto an after-production surface with “PalbondLA35 (produced by NIHON PARKERIZING Corp.)” as prescribed by the maker;further cationic electrodeposition coating (“Powernics 110”: produced byNIPPON PAINT Corp.) was performed in a thickness of 15 μm; and then,after subjecting it to cross cutting, the paint-film swollen width(one-sided) from the cross-cut portion was measured under the corrosiontesting conditions (SAE-J2334, a standard of the society of AmericanAutomobile Industry) after performing the test 300 cycles. Themeasurement results are specified in Table 3.

In order to make the present invention more definite, the examples andcomparatives will be explained while illustrating the electrolyticcome-off curves, which were shown at the time of having the platedlayers come off electrolytically, in FIG. 1-FIG. 8. The electrolyticcome-off curves specify from the beginning of electrolysis to base iron.FIG. 1 illustrates Comparative Example No. 2. FIG. 2 illustratesComparative Example No. 9. FIG. 3 illustrates Example No. 6. FIG. 4illustrates Comparative Example No. 1. FIG. 5 illustrates ComparativeExample No. 10. FIG. 6 illustrates Example No. 9. FIG. 7. illustratesExample No. 10. FIG. 8 illustrates Comparative Example No. 5.

Comparative Example No. 9 illustrated in FIG. 2 is an ordinary alloyedsteel plate, which was made by molten zinc-plating but which was notquenched. In FIG. 2, the portion “A” is a region which exhibits apotential of about −800 mV (vs. SCE) or less, and designates a layerwhich is formed of Fe: 30% by mass or less. As can be understood fromFIG. 2 and Table 2, in Comparative Example No. 9, the plated layer isformed of the “Zn—Fe layer which is formed of Fe: 30% by mass or less”which exhibits a potential of about −800 mV (vs. SCE) or less, namely,the portion “A” alone. In Comparative Example No. 9, since no heatingupon quenching is carried out, the later-described portion “B” is notgenerated. And, after the layer of portion “A” has dissolved, theportion “C,” iron (the test piece's iron substrate), which exhibits apotential of about −560 mV, is exposed.

Comparative Example No. 10 illustrated in FIG. 5 is an ordinary alloyedsteel plate, which was made by molten zinc-plating but which was notquenched. As can be understood from FIG. 5 and Table 2, ComparativeExample No. 10 has the portion “A” and portion “C” similarly toComparative Example No. 9. Since no heating upon quenching is carriedout, the later-described portion “B” is not generated. As describedabove, the portion “A” is a region which exhibits a potential of about−800 mV (vs. SCE) or less, and is a Zn—Fe layer which includes Fe: 30%by mass or less. Therefore, the portion “A” is equivalent to a layer inwhich Fe is less but Zn is more and whose corrosion resistance issatisfactory. The portion “C” is equivalent to the iron substrate, whichconstitutes the test piece (formed-body steel material). Note that,although it is natural, both of them (Comparative Example Nos. 9 and 10)do not become high strength at all as specified in Table 3 because theyare not strengthened by quenching.

Here, FIG. 3 illustrates the electrolytic come-off curve of Example No.6. FIG. 6 illustrates the electrolytic come-off curve of Example No. 9.FIG. 7 illustrates the electrolytic come-off curve of Example No. 10.FIG. 8 illustrates the electrolytic come-off curve of ComparativeExample No. 5. In accordance with Example No. 6, Example No. 9, ExampleNo. 10 and Comparative Example No. 5, the portion “B” exists in additionto the portion “A” and portion “C,” as illustrated in FIG. 3, FIG. 6,FIG. 7 and FIG. 8. Here, the portion “B” exhibits intermediatepotentials from the region of the potential of about −560 mV (vs. SCE)down to the region of the potential of about −800 mV (vs. SCE). Thisportion “B” is a layer, which is generated within the zinc-plated layerby means of the heating upon quenching, and is a “Zn—Fe alloy layer inwhich Zn is less and Fe is a major component (in excess of 30%-by-massFe).” The present invention is such that it is the chief aim to make theportion “A” whose corrosion resistance is good, not the portion “B”whose corrosion resistance is not sufficient, in an amount of 30 g/cm²or more.

In Example No. 6 illustrated in FIG. 3, Example No. 9 illustrated inFIG. 6, Example No. 10 illustrated in FIG. 7, and Comparative ExampleNo. 5 illustrated in FIG. 8, the zinc-plated steel plates, whichcontained the alloying-retarding elements and readily-oxidizing elementsin an amount of 0.16% by mass, were heated to 850° C. in a0.10%-by-volume oxygen atmosphere; and were thereafter quenched whilechanging the holding time only. In Comparative Example No. 5, despitethe fact that the thickness of the steel plate is 1.6 mm, since theholding time is as long as 15 minutes relatively, the “Zn—Fe layer whichis formed of Fe: 30% by mass or less” is 18 g/m², and is less than 30m²/g. From the comparison between Example Nos. 9 and 10 and ComparativeExample No. 5, it is appreciated that, as the heating time becomeslonger, the portion “A” decreases but the portion “B” increases.

As can be understood from Table 2 and Table 3, Comparative Example No. 1is such that an ordinary electrogalvanized steel plate was quenched byheating. In Comparative Example No. 1, since no alloying-retardingelements and readily-oxidizing elements (Al, Si) exist so that, althoughthe heating conditions are lax, no portion “A” exists but it becomes theportion “B” alone, the corrosion resistance is not demonstrated, and thevolatilization of zinc is great as well.

As can be understood from Table 2 and Table 3, Comparative Example No. 2is such that an alloyed molten zinc-plated steel plate was quenched byheating. In Comparative Example No. 2, since the alloying-retardingelements and readily-oxidizing elements are less (Al: 0.10% by mass) andthe alloying treatment is carried out in advance, no portion “A” existsbut the growth of the portion “B” is remarkable; and the corrosionresistance is demonstrated much worse than that of Comparative ExampleNo. 1, and the volatilization of zinc is great as well.

As specified in Table 3, in Example Nos. 1-28 according to the presentinvention, the “layer in which Zn is a major component and which isformed of Fe: 30% by mass or less (Fe:9-23% by mass)” (equivalent to theportion “A”) is made in an amount of 30 g/m² or more (31-223 g/m²). Insuch Example Nos. 1-28, the swollen width was small, and accordingly thecorrosion resistance was satisfactory. Incidentally, Example No. 5exhibits Fe: 9% by mass, and Example No. 9 exhibits Fe: 23% by mass.Example No. 5 exhibits 223 g/m², and Example No. 10 exhibits 31 g/m².

Moreover, an alloy layer (equivalent to the portion “B”), which isgenerated by means of heating in quenching and in which Fe is a majorcomponent, is formed in an amount of 5 g/m² or more (5-155 g/m²). Asabove, in order to demonstrate the after-quenching corrosion resistancein zinc-system-plated steel materials, the present invention has beencompleted by leaving the portion “A,” which has the anticorrosioneffect, in a predetermined amount or more, by means of the suppressionand control of the generation of portion “B,” which results from heatingin quenching, and the suppression of the volatilization of zinc.

TABLE 1 Steel Species No. C Si Mn P S Al Ti Cr Ni Nb Mo V Zr B N “A”0.21 0.15 1.30 0.02 0.01 0.05 0.03 0.3 0.02 0.05 0.1 0.05 0.01 20 ppm 30ppm “B” 0.18 0.15 0.70 0.01 0.01 0.08 0.02 0.2 0.8 0.07 0.4 0.01 0.02 10ppm 40 ppm “C” 0.11 0.20 1.10 0.01 0.01 0.07 0.01 0.2 0 0.02 0.3 0.010.02 10 ppm 30 ppm “D” 0.30 0.25 1.60 0.02 0.01 0.05 0.03 0.3 0.02 0.050.1 0.05 0.01 20 ppm 30 ppm Except B and N, % by mass

TABLE 2 Before Heating Inner Plate Plated Zn Steel Thickness PlatingAmount Amount Composition (%) No. Species mm Method g/m² g/m² Fe % Al SiMg Ti Ce Comp. 1 “A” 1.6 Electro- 60 60 <1% 0.00 Ex. Galvanizing Comp. 2“A” 1.6 Alloyed 62 56 9 0.10 Ex. Molten Zinc Comp. 3 “B” 1.0 Molten 3131 <1% 0.20 Ex. Zinc Ex. 1 “B” 1.0 Molten 42 42 <1% 0.20 Zinc Ex. 2 “A”1.6 Molten 59 59 <1% 0.20 Zinc Ex. 3 “B” 1.8 Molten 88 88 <1% 0.20 ZincEx. 4 “C” 2.3 Molten 122 122 <1% 0.20 Zinc Ex. 5 “C” 3.2 Molten 265 264<1% 0.20 Zinc Comp. 4 “A” 1.6 Molten 92 92 <1% 0.16 Ex. Zinc Ex. 6 “A”1.6 Molten 92 92 <1% 0.16 Zinc Ex. 7 “A” 1.6 Molten 92 92 <1% 0.16 ZincEx. 8 “A” 1.6 Molten 92 92 <1% 0.16 Zinc Ex. 9 “A” 1.6 Molten 92 92 <1%0.16 Zinc Ex. 10 “A” 1.6 Molten 92 92 <1% 0.16 Zinc Comp. 5 “A” 1.6Molten 92 92 <1% 0.16 Ex. Zinc Ex. 11 “A” 1.6 Molten 92 92 <1% 0.16 ZincComp. 6 “A” 1.6 Molten 92 92 <1% 0.16 Ex. Zinc Ex. 12 “A” 1.6 Molten 9292 <1% 0.16 Zinc Comp. 7 “A” 1.6 Molten 92 92 <1% 0.16 Ex. Zinc Ex. 13“C” 1.2 Molten 61 60 <1% 1.00 Zinc Ex. 14 “C” 1.2 Molten 61 58 <1% 5.00Zinc Ex. 15 “C” 1.2 Molten 61 54 <1% 11.00 Zinc Ex. 16 “C” 1.2 Molten 6149 <1% 20.00 Zinc Ex. 17 “C” 1.2 Molten 90 90 <1% 0.00 0.3 Zinc Ex. 18“C” 1.2 Molten 90 91 <1% 0.10 0.2 Zinc Ex. 19 “C” 1.2 Molten 90 89 <1%0.20 0.1 Zinc Ex. 20 “C” 1.2 Molten 90 92 <1% 0.00 0.2 0.1 Zinc Ex. 21“C” 1.2 Molten 90 88 <1% 0.20 0.3 Zinc Ex. 22 “C” 1.2 Molten 90 88 <1%0.20 0.1 0.1 Zinc Ex. 23 “C” 1.2 Molten 90 86 <1% 0.20 Zinc Ex. 24 “C”1.2 Molten 90 92 <1% 0.16 Zinc Ex. 25 “C” 1.2 Molten 90 91 <1% 0.16 ZincEx. 26 “C” 1.2 Molten 90 93 <1% 0.16 Zinc Ex. 27 “C” 1.8 Molten 125 125<1% 0.22 Zinc Ex. 28 “D” 0.8 Molten 118 118 <1% 0.21 Zinc Comp. 8 “A”1.6 Electro 60 60 <1% 0.00 Ex. Galvanizing Comp. 9 “A” 1.6 Alloyed 62 569 0.10 Ex. Molten Zinc Comp. 10 “A” 1.6 Molten 59 59 <1% 0.20 Ex. ZincComp. 11 “B” 1.8 Molten 88 88 <1% 0.20 Ex. Zinc Comp. 12 “C” 2.3 Molten122 122 <1% 0.20 Ex. Zinc Comp. 13 “D” 0.8 Molten 122 122 <1% 0.21 Ex.Zinc Upper-layer Plating Heating Condition Plated Heating Holding PlatedAmount Heating Temp. Time Heating Oxygen No. Species g/m² Method ° C.min. Atmosphere Concentration Comp. 1 Electric 820 0 Air 20% Ex. Furnace(oxygen: 20%) Comp. 2 Electric 820 0 Air 20% Ex. Furnace (oxygen: 20%)Comp. 3 Electric 820 0 Air 20% Ex. Furnace (oxygen: 20%) Ex. 1 Electric820 0 Air 20% Furnace (oxygen: 20%) Ex. 2 Electric 900 0 Air 20% Furnace(oxygen: 20%) Ex. 3 Electric 900 0 Air 20% Furnace (oxygen: 20%) Ex. 4Electric 900 0 Air 20% Furnace (oxygen: 20%) Ex. 5 Electric 900 0 Air20% Furnace (oxygen: 20%) Comp. 4 Gas 700 5 Air 0.10%   Ex. FurnaceRatio = 1 Ex. 6 Gas 850 0 Air 0.10%   Furnace Ratio = 1 Ex. 7 Gas 850 1Air 0.10%   Furnace Ratio = 1 Ex. 8 Gas 850 3 Air 0.10%   Furnace Ratio= 1 Ex. 9 Gas 850 5 Air 0.10%   Furnace Ratio = 1 Ex. 10 Gas 850 10 Air0.10%   Furnace Ratio = 1 Comp. 5 Gas 850 15 Air 0.10%   Ex. FurnaceRatio = 1 Ex. 11 Gas 950 1 Air 0.10%   Furnace Ratio = 1 Comp. 6 Gas1050 1 Air 0.10%   Ex. Furnace Ratio = 1 Ex. 12 Gas 950 1 Air  1%Furnace Ratio = 1.1 Comp. 7 Gas 950 1 Air 0.1 ppm Ex. Furnace Ratio =0.9 Ex. 13 Gas 950 1 Air  1% Furnace Ratio = 1.1 Ex. 14 Gas 950 1 Air 1% Furnace Ratio = 1.1 Ex. 15 Gas 950 1 Air  1% Furnace Ratio = 1.1 Ex.16 Gas 950 1 Air  1% Furnace Ratio = 1.1 Ex. 17 Induction 900 3 Air 20%Heating (oxygen: 20%) Ex. 18 Induction 900 3 Air 20% Heating (oxygen:20%) Ex. 19 Induction 900 3 Air 20% Heating (oxygen: 20%) Ex. 20Induction 900 3 Air 20% Heating (oxygen: 20%) Ex. 21 Induction 900 3 Air20% Heating (oxygen: 20%) Ex. 22 Induction 900 3 Air 20% Heating(oxygen: 20%) Ex. 23 Ni 5 Induction 900 3 Air 20% Heating (oxygen: 20%)Ex. 24 Ni 2 Induction 900 3 Air 20% Heating (oxygen: 20%) Ex. 25 Co 5Induction 900 3 Air 20% Heating (oxygen: 20%) Ex. 26 Fe 3 Induction 9003 Air 20% Heating (oxygen: 20%) Ex. 27 Infrared 800 0 Air 20% Heating(oxygen: 20%) Ex. 28 Infrared 900 2 Air 20% Heating (oxygen: 20%) Comp.8 None Ex. Comp. 9 None Ex. Comp. 10 None Ex. Comp. 11 None Ex. Comp. 12None Ex. Comp. 13 None Ex.

TABLE 3 Fe < 30% Zn—Fe 30% < Fe Zn—Fe Alloy Layer Alloy Layer CorrosionAlloy Inner Alloy Inner Zn Volatil- Resistance Layer (Zn Layer (Znization Swollen Cooling Amount Amount) Amount Amount) Amount widthStrength Method g/m² g/m² Fe % g/m² g/m² Fe % g/m² mm MPa Comp. 1 Water0 104 47 55 13 30 1543 Ex. Cooling Comp. 2 Water 0 115 34 70 22 45 1555Ex. Cooling Comp. 3 Water 21 16 22 27 15 46 0 12 1330 Ex. Cooling Ex. 1Water 32 26 19 31 16 48 0 3 1328 Cooling Ex. 2 Gas 50 42 16 34 17 51 0 21515 Cooling Ex. 3 Gas 79 69 13 41 19 53 0 2 1288 Cooling Ex. 4 Gas 10292 10 67 30 55 0 1 1026 Cooling Ex. 5 Gas 223 203 9 147 62 58 0 1 1033Cooling Comp. 4 Die 68 61 11 61 31 49 0 1 630 Ex. Cooling Ex. 6 Die 8574 13 36 18 50 0 1 1515 Cooling Ex. 7 Die 74 63 15 60 29 52 0 1 1520Cooling Ex. 8 Die 59 48 18 91 43 52 0 2 1533 Cooling Ex. 9 Die 48 37 22116 54 53 0 2 1540 Cooling Ex. 10 Die 31 24 23 155 68 56 0 3 1535Cooling Comp. 5 Die 18 14 23 177 78 56 0 15 1538 Ex. Cooling Ex. 11 Die73 62 15 63 30 53 0 1 1536 Cooling Comp. 6 Die 5 4 21 95 40 58 48 231541 Ex. Cooling Ex. 12 Die 75 64 15 57 28 51 0 1 1533 Cooling Comp. 7Die 22 18 20 65 31 52 43 13 1532 Ex. Cooling Ex. 13 Die 51 43 15 39 1756 0 2 1049 Cooling Ex. 14 Die 53 45 15 29 13 56 0 2 1052 Cooling Ex. 15Die 55 46 16 18 8 55 0 2 1045 Cooling Ex. 16 Die 55 47 15 5 2 56 0 21043 Cooling Ex. 17 Water 62 53 15 74 37 50 0 1 1081 Cooling Ex. 18Water 65 56 14 69 35 49 0 1 1077 Cooling Ex. 19 Water 64 54 16 72 35 510 1 1075 Cooling Ex. 20 Water 59 50 15 85 42 51 0 1 1082 Cooling Ex. 21Water 60 52 13 75 36 52 0 1 1088 Cooling Ex. 22 Water 59 51 14 75 37 500 1 1079 Cooling Ex. 23 Die 53 42 21 96 44 54 0 2 1051 Cooling Ex. 24Die 55 45 19 105 47 55 0 2 1047 Cooling Ex. 25 Die 53 42 20 108 49 55 02 1047 Cooling Ex. 26 Die 54 44 19 107 49 54 0 2 1055 Cooling Ex. 27 Gas110 96 13 61 29 52 0 1 813 Cooling Ex. 28 Water 103 84 18 78 34 57 0 11980 Cooling Comp. 8 60 60 0 0 — 2 613 Ex. Comp. 9 62 56 9 0 — 1 615 Ex.Comp. 10 59 59 0 0 — 1 613 Ex. Comp. 11 88 88 0 0 — 1 488 Ex. Comp. 12122 122 0 0 — 1 451 Ex. Comp. 13 122 122 0 0 — 1 615 Ex.

Note that, before the quenching treatment, it is possible as well tothink of a measure of forming a barrier layer, which comprises anoxidized film, on the zinc-plated layer of a steel material; in thiscase, however, when heating the steel material to a quenchingtemperature, or when heating it to the quenching temperature and holdingit thereat, there is a fear that cracks might generate considerably inthe zinc-plated layer because of the thermal expansion differencebetween the barrier layer, which has been formed originally on the steelmaterial, and the zinc-plated layer. In this case, due to the cracks,the fear that the volatilization amount of zinc increases is highlylikely; and, after the quenching treatment, it is not necessarilysufficient in order to obtain the plated layer whose corrosionresistance is good. In this a case, it is believed that the proportionof portion “A” is not so much as that of the present invention; and thatthe proportion of portion “B” becomes greater compared with that of thepresent invention.

It is possible to grasp the following technical ideas as well from theaforementioned descriptions.

-   -   A high-strength quenched formed body, which is good in corrosion        resistance, being characterized in that: it comprises an        after-quenching formed-body steel material; and a zinc-plated        layer, which is disposed on the surface of said formed-body        steel material, and which is subjected to a quenching treatment        along with said formed-body steel material; said zinc-plated        layer comprises a layer in which Zn is a major component, and        which is formed of Fe: 30% by mass or less; and said layer, in        which Zn is a major component and which is formed of Fe: 30% by        mass or less, is made in an amount of 30 g/m² or more.    -   A process for producing a high-strength quenched formed body,        process in which the following are performed sequentially: a        step of preparing a formed-body steel material, on which a        zinc-plated layer is coated, zinc-plated layer which contains        each of Al and Si, which have alloying-retarding function and        readily-oxidizing function, independently or compositely in an        amount of 0.15% by mass or more; and a step of quenching said        formed-body steel material after heating it to a quenching        temperature region in an oxidizing atmosphere of oxygen: 0.1% by        volume or more, thereby producing a high-strength quenched        formed body, which comprises said formed-body steel material        after the quenching; and a layer being disposed on the surface        of said formed-body steel material, the layer being subjected to        a quenching treatment along with said formed-body steel        material, the layer being made from Zn as a major component, and        the layer being formed of Fe: 30% by mass or less; and said        layer in which Zn is a major component and which is formed of        Fe: 30% by mass or less is made in an amount of 30 g/m² or more.        As for the quenching temperature region, 800-950° C. can be        named.

The present invention is not limited to the examples, which aredescribed above and are illustrated in the drawings, alone, but can beperformed while modifying them suitably within ranges not-departing fromthe spirit or scope thereof.

INDUSTRIAL APPLICABILITY

The present invention can be utilized for high-strength quenched formedbodies and production processes for the same.

1. A process for producing a quenched formed body comprising the following performed sequentially: preparing a formed-body steel material on which a zinc-plated layer is coated, wherein the zinc-plated layer contains at least Al and has an alloying-retarding function and a readily-oxidizing function, wherein Al is present in the zinc-plated layer in an amount of from 0.15% to 20% by mass wherein % by mass is based on the total weight of the zinc-plated layer as 100% by mass, wherein said zinc-plated layer is coated onto the formed-body steel material by molten zinc-plating; and heating the formed-body steel material coated with the zinc-plated layer in a single step to a temperature of from 800° C. to 950° C. in an oxidizing atmosphere comprising oxygen in an amount of 0.1% by volume or more, then quenching said formed-body steel material coated with the zinc-plated layer by continuously cooling in one step the heated formed-body steel material without heating, thereby producing: a quenched formed body comprising an after-quenching formed-body steel material having the zinc-plated layer disposed on the surface, wherein the zinc-plated layer has been subjected to the quenching along with said formed-body steel material, wherein the zinc-plated layer comprises Zn as a major component and 30% by mass or less of Fe; and wherein said zinc-plated layer which comprises Zn as a major component and 30% by mass or less of Fe is present in an amount of 30 g/m² or more; and wherein the zinc-plated layer comprises Si in an amount of 0.2% by mass or more.
 2. The process of claim 1, wherein the cooling of the quenching of said formed-body steel material is carried out by water cooling, die cooling or gas cooling.
 3. The process of claim 1, wherein after the quenching, said zinc-plated layer which comprises Zn as a major component and 30% by mass or less of Fe, exhibits a potential of −800 mV (vs. SCE) when a saturated calomel electrode (SCE) is a reference electrode.
 4. The process of claim 1, wherein prior to the heating of the quenching treatment, said zinc-plated layer contains at least one member selected from the group consisting of Mg, Ti, rare-earth elements, Fe, Ni and Co, in an amount of 2.0% by mass or less.
 5. The process of claim 1, wherein the zinc-plated layer further comprises at least one selected from the group consisting of Mg, Ti and Ce, in an amount of 0.1% by mass or more.
 6. The process of claim 1, wherein the zinc-plated layer comprises a total amount of Al and Si of 6% by mass or less.
 7. The process of claim 1, wherein the formed-body steel material after quenching is coated with the layer in an amount of 90 g/m² or more.
 8. The process of claim 1, wherein the oxidizing atmosphere comprises oxygen in an amount of 1% by volume or more.
 9. The process of claim 1, wherein the oxidizing atmosphere comprises oxygen in an amount of 20% by volume or more.
 10. The process of claim 1, wherein the zinc-plated layer formed after the heating and the quenching 19% by mass or less of Fe.
 11. The process of claim 1, wherein the zinc-plated layer formed after the heating and the quenching 10% by mass or less of Fe.
 12. A process for producing a quenched formed body comprising the following performed sequentially: preparing a formed-body steel material on which a zinc-plated layer is coated, wherein the zinc-plated layer contains at least Al and has an alloying-retarding function and a readily-oxidizing function, wherein Al is present in the zinc-plated layer in an amount of from 0.15% to 20% by mass wherein % by mass is based on the total weight of the zinc-plated layer as 100% by mass, wherein said zinc-plated layer is coated onto the formed-body steel material by molten zinc-plating; and heating the formed-body steel material coated with the zinc-plated layer in a single step to a temperature of from 800° C. to 950° C. in an oxidizing atmosphere comprising oxygen in an amount of 0.1% by volume or more, then quenching said formed-body steel material coated with the zinc-plated layer by continuously cooling in one step the heated formed-body steel material without heating, thereby producing: a quenched formed body comprising an after-quenching formed-body steel material having the zinc-plated layer disposed on the surface, wherein the zinc-plated layer has been subjected to the quenching along with said formed-body steel material, wherein the zinc-plated layer comprises Zn as a major component and 30% by mass or less of Fe; and wherein said zinc-plated layer which comprises Zn as a major component and 30% by mass or less of Fe is present in an amount of 30 g/m² or more; and wherein the zinc-plated layer comprises Si in an amount of 0.3% by mass or more.
 13. The process of claim 12, wherein the cooling of the quenching of said formed-body steel material is carried out by water cooling, die cooling or gas cooling.
 14. The process of claim 12, wherein after the quenching, said zinc-plated layer which comprises Zn as a major component and 30% by mass or less of Fe, exhibits a potential of −800 mV (vs. SCE) when a saturated calomel electrode (SCE) is a reference electrode.
 15. The process of claim 12, wherein prior to the heating of the quenching treatment, said zinc-plated layer contains at least one member selected from the group consisting of Mg, Ti, rare-earth elements, Fe, Ni and Co, in an amount of 2.0% by mass or less.
 16. The process of claim 12, wherein the zinc-plated layer further comprises at least one selected from the group consisting of Mg, Ti and Ce, in an amount of 0.1% by mass or more.
 17. The process of claim 12, wherein the zinc-plated layer comprises a total amount of Al and Si of 6% by mass or less.
 18. The process of claim 12, wherein the formed-body steel material after quenching is coated with the layer in an amount of 90 g/m² or more.
 19. The process of claim 12, wherein the oxidizing atmosphere comprises oxygen in an amount of 1% by volume or more.
 20. The process of claim 12, wherein the oxidizing atmosphere comprises oxygen in an amount of 20% by volume or more.
 21. The process of claim 12, wherein the zinc-plated layer formed after the heating and the quenching 19% by mass or less of Fe.
 22. The process of claim 12, wherein the zinc-plated layer formed after the heating and the quenching 10% by mass or less of Fe.
 23. A process for producing a quenched formed body comprising the following performed sequentially: preparing a formed-body steel material on which a zinc-plated layer is coated, wherein the zinc-plated layer contains at least Al and has an alloying-retarding function and a readily-oxidizing function, wherein Al is present in the zinc-plated layer in an amount of from 0.15% to 20% by mass wherein % by mass is based on the total weight of the zinc-plated layer as 100% by mass, wherein said zinc-plated layer is coated onto the formed-body steel material by molten zinc-plating; and heating the formed-body steel material coated with the zinc-plated layer in a single step to a temperature of from 800° C. to 950° C. in an oxidizing atmosphere comprising oxygen in an amount of 0.1% by volume or more, then quenching said formed-body steel material coated with the zinc-plated layer by continuously cooling in one step the heated formed-body steel material without heating, thereby producing: a quenched formed body comprising an after-quenching formed-body steel material having the zinc-plated layer disposed on the surface, wherein the zinc-plated layer has been subjected to the quenching along with said formed-body steel material, wherein the zinc-plated layer comprises Zn as a major component and 30% by mass or less of Fe; and wherein said zinc-plated layer which comprises Zn as a major component and 30% by mass or less of Fe is present in an amount of 30 g/m² or more; and wherein the zinc-plated layer comprises Si in an amount of 0.6% by mass or more.
 24. The process of claim 23, wherein the cooling of the quenching of said formed-body steel material is carried out by water cooling, die cooling or gas cooling.
 25. The process of claim 23, wherein after the quenching, said zinc-plated layer which comprises Zn as a major component and 30% by mass or less of Fe, exhibits a potential of −800 mV (vs. SCE) when a saturated calomel electrode (SCE) is a reference electrode.
 26. The process of claim 23, wherein prior to the heating of the quenching treatment, said zinc-plated layer contains at least one member selected from the group consisting of Mg, Ti, rare-earth elements, Fe, Ni and Co, in an amount of 2.0% by mass or less.
 27. The process of claim 23, wherein the zinc-plated layer further comprises at least one selected from the group consisting of Mg, Ti and Ce, in an amount of 0.1% by mass or more.
 28. The process of claim 23, wherein the zinc-plated layer comprises a total amount of Al and Si of 6% by mass or less.
 29. The process of claim 23, wherein the formed-body steel material after quenching is coated with the layer in an amount of 90 g/m² or more.
 30. The process of claim 23, wherein the oxidizing atmosphere comprises oxygen in an amount of 1% by volume or more.
 31. The process of claim 23, wherein the oxidizing atmosphere comprises oxygen in an amount of 20% by volume or more.
 32. The process of claim 23, wherein the zinc-plated layer formed after the heating and the quenching 19% by mass or less of Fe.
 33. The process of claim 23, wherein the zinc-plated layer formed after the heating and the quenching 10% by mass or less of Fe. 